Concerning this most important element in which we live and move and have our being, Pliny, in the first century of our era, wrote as follows: "It is time to consider the other marvels of the heavens; thus our fathers called that immense space where flows the vital fluid to which we give the name of air, and which is not apparent to the senses because of its great rarity. There clouds form, thunder and lightning also; it is the region of tempests and of whirlwinds; from there fall rains, hail, and hoar frost; from there come all those phenomena, astonishing and often disastrous, which follow the combat of Nature with herself…. The sun's rays strike the earth on all sides, warming and strengthening it; they are reflected and detach all the particles they can carry away; vapours descend and rise again; the winds come empty and return laden with spoil; animals breathe in from above this vital fluid which animates them, and the earth sends it back to its source as if she would fill the void by this means. So, by Nature acting everywhere and in all directions there results an apparent discord from which is born the harmony of the Universe; it is this general movement which puts all things in their places; some are preserved by the destruction of others; all move, all act, the struggle is continual, if it ceased an instant everything would fall into chaos…."
From the earliest times, as far back as history extends, we find mankind interested in meteorological phenomena. This appears natural if we consider the importance of the weather to the ancient pastoral nations, which, from the open-air life and keen perceptive faculties of their people, were well fitted to study natural phenomena. The beauty and grandeur of many of the phenomena occurring in the atmosphere, and the curiosity excited concerning their causes, probably contributed to interest people in them. Meteorology appears to have been first treated systematically, as distinct from astronomy and astrology, by the Greek philosopher, Aristotle, more than 2000 years ago. The word "meteor," derived from the Greek "elevated," was applied to certain phenomena having their origin in the atmosphere. These were classified into aërial, aqueous, and luminous meteors, and were all included in the term Meteorology. In his treatise by this name Aristotle gave a more detailed account of them than any preceding or contemporary writer, and Theophrastus, his pupil, wrote two books on the winds and on the signs of rain, which have been translated into Latin and English. About the same period Aratus incorporated the current weather proverbs in his poem, Diosemeia. The Greek historians and poets frequently alluded to atmospheric phenomena, and their example was followed by the Romans, of whom Pliny has been quoted.
No doubt the desire to ascend into the air always possessed man, but owing to the awe with which mountains seem to have inspired the ancients, there is rarely mention in their writings of climbing mountains, or of the physiological effects which could hardly have failed to be apparent upon high summits. Citing one of the few existing narratives, Aristotle relates: "Those which ascend to the top of the mountain Olympus could not keep themselves alive without carrying with them wet sponges, by whose assistance they could respire in that air otherwise too thin for respiration." This mountain of less than 10,000 feet was said to be so high that it never rained on its summit, where, it was supposed, the air was always still. A still higher mountain, easily accessible to the ancient world, and which we know was ascended, is Etna.
Concerning the progress of meteorology, from the time of the ancient Romans to the revival of knowledge in Europe, there is little to say except that during the middle ages meteorology, like other learning, was confined to the monasteries. Speculations were current as to the extent of the atmosphere until, in the middle of the eleventh century, Alhazen, a learned Arab, computed from the duration of twilight that the atmosphere extended nineteen leagues above the earth. The same method was applied with more precision by Tycho Brahe, Kepler, and other astronomers of the sixteenth and seventeenth centuries. The earliest weather chronicles were probably noted by monks from time to time in almanacks or missals, although when this was done first we do not know. The oldest daily chronicles of the weather extant are those kept by William Merle in Oxford from 1337 to 1344. We owe it to the late Mr. Symons, the English meteorologist and bibliophile, that this MS. and many other old records have been brought to light and published. Dr. Hellmann has done even more in Germany, and this historical research is evidence of the growing importance of the science of meteorology.
With the advent of the age of geographical discovery it was seen that the climatic features of our globe depend chiefly upon distance from the equator, proximity to the ocean, and height above it. In the tropics especially, the luxuriant vegetation, which diminishes on mountain slopes and higher up gives place to snow, must have been visible proof of the decrease of temperature with altitude, for, as Professor Daniell remarked, mountains are a gigantic registering thermometer having for the freezing-point the line of perpetual snow. The invention of instruments for measuring temperature and barometric pressure made possible the quantitative observations that have supplied the data for deducing the laws governing the atmosphere. The oldest meteorological instrument is, no doubt, the weather or wind-vane, which had its origin before the Christian era. The next oldest is the hygrometer, or instrument for measuring moisture in the air, the form which acts by absorption dating from the middle of the fifteenth century, and the condensation hygrometer being a century younger. Next in chronological order comes the rain-gauge, which appears to have been used by Castelli, a friend of Galileo, in the year 1639. The history of that important instrument, the thermometer, is obscure, but it is certain that Galileo in Padua used an air-thermometer in the latter part of the sixteenth century, which Rey, a French physician, filled with liquid in 1631. This thermometer, as well as other physical instruments, was perfected by members of the Accademia del Cimento at Florence. These instruments are described in Saggi di Naturali Esperienza, written in 1666, and translated into Latin and English. The Florentine thermometers had one fixed point, that of freezing water, and contained either spirits or mercury. In 1724 Fahrenheit, in Danzig, fixed three points on the scale of the mercurial thermometer, viz. the cold produced by ice and sal-ammoniac which he called 0°, freezing water or 32°, and the heat of the human blood which he assumed to be 96°. This thermometric scale, having 180° between freezing and boiling water, and that of Celsius, with 100°, are the only ones in scientific use to-day. It is a remarkable fact in the history of thermometers that neither of these thermometers remained in the country where it was invented; thus the thermometer of Fahrenheit, a German, came into use exclusively in England and her colonies, while that of Celsius, a Swede, is now used on the continent of Europe except in Germany, where the thermometer of Réaumur, a Frenchman, is still in popular use. Of the four fundamental meteorological instruments, the barometer was the last invented. Aristotle had suspected that air had weight, but it was not demonstrated until the middle of the seventeenth century, when the old axiom "that Nature abhors a vacuum" was replaced by the rational explanation, given by Galileo and Torricelli, his pupil, why water will not rise in a suction pump more than thirty-two feet. In 1643 Torricelli executed this famous experiment: he took a glass tube, sealed at one end, and filled it with mercury, then, closing the open end with his finger, he inverted it in a basin of mercury. The mercury fell to about thirty inches, which was recognized to be the weight of a column having the area of the tube and of the height of the atmosphere. The application of the barometer was due to Blaise Pascal, who repeated at Rouen Torricelli's experiment with a much longer tube filled with water, which being thirteen times lighter than mercury, stood thirteen times higher, or thirty-two feet, in the tube. Pascal, being himself at Paris in 1648, got his brother-in-law Perier to carry a barometric tube filled with mercury to the top of the Puy de Dôme, a mountain in Auvergne rising about 3500 feet above the city of Clermont. The mercury fell in the tube with the ascent, and at the top of the mountain it stood some three inches lower than at the base, showing that the lower layers of the atmosphere are denser than the upper. Pascal repeated the experiment on the Tower of St. Jacques in Paris, and it is interesting to note that more than two hundred years afterwards, meteorological stations were established both there and on the Puy de Dôme. It was soon perceived that not only did the level of the mercury in the tube change with height, but that it oscillated continually at the same place, and from its observed relation to the state of the weather its name "weather-glass" is derived. In 1650 the weight of the air was demonstrated in another manner by Otto von Guericke, burgomaster of Magdeburg, who by means of an air-pump of his invention performed the experiment, which Aristotle had tried unsuccessfully, of weighing a vessel full of air and the same vessel exhausted of air. He also showed the pressure of the air in all directions by the famous experiment of the Magdeburg hemispheres, which, being hollow, were placed together, and after the air was exhausted from the sphere so formed sixteen horses were unable to pull them apart. Soon afterwards Robert Boyle experimented further upon the weight and "spring of the air," as he called it, and gave the name to the barometer. Both Boyle in England and Mariotte in France discovered the law, bearing indifferently their names, that the pressure of gases is proportional to their density. Halley, a few years later, showed that the rate of decrease in pressure differed from the rate of increase in height, and developed formulæ for measuring heights by the barometer, which were afterwards perfected by Laplace. Knowing the heights of the barometer at a high and at a low-level station, and the mean temperature of the air lying between them, it is possible to compute accurately the difference of height of the two stations, or, conversely, given this height, the difference in barometric pressure can be calculated. By the middle of the seventeenth century the most important meteorological instruments had been invented, and not only can Italy claim to be their birthplace, but the Grand Duke Ferdinand II., whose brother Leopold founded the Accademia del Cimento, distributed the new instruments in Italy and even beyond the Alps, so that in 1654 observations several times a day were begun at a dozen stations. The observations in Florence from 1650 to 1670 were preserved and constitute the commencement of instrumental meteorology.
It was the conquest of Peru which, by leading men over the high passes of the Andes, first brought them to great heights, but although we find mention in the history of the expeditions of the so-called mountain sickness, caused by fatigue as well as by cold and rarefied air, it does not appear that scientific observations were made. Therefore, while it must be assumed that the atmospheric conditions at considerable altitudes were familiar to travellers, yet not until the middle of the last century did Bouguer, one of three French Academicians sent to Peru on a geodetic mission, fix the height of the freezing point in various latitudes, after observing that the temperature fell below freezing at night upon the mountains near the equator. During the latter part of the century, Kirwan, an English chemist, calculated the temperature for various parallels of latitude, and in 1817 Alexander von Humboldt, after a voyage around the world, published his isothermal lines, or lines of equal temperature on the surface of the globe, by which he showed that the deviation from the normal, or calculated, temperature arose from the distribution of land and water, and from the geographical relief of the former. This work of von Humboldt formed the basis of all subsequent studies in comparative climatology. Meanwhile chemistry had kept pace with physics, and in 1774 the old theory, that air was one of the four elements from which all things originated, was rendered untenable by Priestley, who proved that oxygen gas, which he discovered, was a constituent part of air. The other constituent, nitrogen, formerly called azote from its destructiveness to life, was discovered soon afterwards, and its proportion in the air determined by the French chemist, Lavoisier.
In 1783 man became possessed of the long-sought-for means of rising freely in the air, and he speedily availed himself of it. The first balloons, filled with heated air, were called Montgolfières from the inventors, the brothers Montgolfier, living in Annonay, France. After animals had been sent up attached to one, Pilâtre de Rozier ventured to ascend in the aerostatic machine, first tethered captive but then set free, and before the close of the year a balloon, filled with hydrogen gas, or "inflammable air" as it was called, carried M. Charles 9000 feet above Paris. During more than a century the balloon has been the most important agent for the exploration of the atmosphere, and yet, notwithstanding the courage and devotion to science of the early aeronauts, their ascents with unsuitable instruments furnished much discordant and erroneous data. Some of the most remarkable balloon voyages and the modern methods of sounding and dredging the atmosphere, to borrow terms from the exploration of the ocean, will be described in two future chapters.
Perhaps the chief reason for the slow progress of meteorology to the status of a science is the variable character of its phenomena with the place of observation. In this respect it differs from astronomy, which was more easily cultivated in the restricted ancient world. Only after many years of observation at different places had contributed a foundation for climatology was it realized that man, in his relation to the atmosphere, resembled marine organisms confined to the bottom of the ocean, and that in order to discover the true conditions of the atmosphere it was necessary to observe them at considerable heights. In the last century the highest point at which physical observations had been made was the summit of Mont Blanc, less than 16,000 feet above the sea. The ascent of this mountain was first accomplished in 1787 by H. B. De Saussure and his guides with much difficulty and suffering, and the observations, abridged and rendered less accurate by the fatigue and sickness of De Saussure, were also influenced by the proximity of the mountain itself. In 1802 von Humboldt and Bonpland reached a height of about 18,000 feet in the Andes, where they made important observations. The ascent of man was rapid during the first years of the nineteenth century, for in 1804 Gay-Lussac rose in a balloon, without exertion or discomfort, to the height of 23,000 feet, and there made observations which were assumed to give the true atmospheric conditions. After an active campaign the conquest of the air by balloons was temporarily abandoned, and the field was left free to the mountaineer. But to-day supremacy rests with the aeronaut, for no one has succeeded in getting higher than 24,000 feet on a mountain, while the aeronaut has exceeded this altitude by a mile without great hardship, and lately has sent his unmanned balloons twice as high as the loftiest mountains.
[Plate I.], headed The Exploration of the Atmosphere, represents a vertical section of the lower portion of our atmosphere. On the right is a scale of miles above the sea, and on the left is a scale of barometric pressures corresponding to the height. The right-hand half of the diagram shows the eastern hemisphere with the Himalaya mountains, the left-hand half the western hemisphere with the Andes. There are seen the heights of the different kinds of clouds, measured at Blue Hill, as described in the next chapter; the highest meteorological stations, those on Mont Blanc and El Misti in Peru; the highest permanently inhabited place, which is a monastery in Thibet; and the greatest height to which man has climbed, namely, in the Andes. The heights at which observations have been made in balloons, carrying observers, or only recording instruments, may be compared with the height attained by the Blue Hill kites, to be described hereafter. Other altitudes can be noted, such as the height of the snow-line on various mountains, and as a thousand-foot measure, the Eiffel Tower in Paris, the tallest structure erected by man, may be used.